Electro-magnetic expulsion de-icing system

ABSTRACT

The present invention is a de-icing system that uses electro-magnetic actuators mounted within the airfoil of an aircraft to effect de-icing. Advantageously, the actuators have low-energy requirements. Each actuator includes conductive strips fabricated on a flexible dielectric sheet. The conductive strips are wound into coils with the actuators shaped into a flattened elongated tube. The axis of the winding coils is coincident with the longitudinal axis of the elongated tube. Low adhesive coatings may be used to enhance the efficiency of the electro-magnetic coils in expulsing the ice accretion.

STATEMENT REGARDING GOVERNMENT FUNDED RESEARCH

This invention was made under Government support under contract No. NAS3-27683 awarded by the National Aeronautics and Space Administration and under contract No. DAAH01-94-C-R146 awarded by the Advanced Research Projects Agency (DOD). The United States Government may have certain rights in this invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No. 08/449,628, entitled "Electro-Magnetic Expulsion De-Icing System," filed May 24, 1995 now U.S. Pat. No. 5,782,435, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to ice-protection systems and, more particularly, relates to de-icing systems employing an electro-magnetic de-icer.

BACKGROUND OF THE INVENTION

Aircraft icing is known to occur on the aircraft exterior surface when the aircraft is airborne as well as when the aircraft is stationary on the ground. Airbone aircraft icing generally occurs on the leading edges of the airfoil when the surface temperature is at or below freezing. Unfortunately, this can occur at any time of the year when there is moisture or precipitation and when the aircraft is at or above the freezing altitude. Such aircraft ice formation or accretion can have deleterious effects on flight performance: lift decreases, thrust falls off, drag and weight increase, and stall speed dramatically increases. In recent years, undetected airborne icing has contributed to a number of catastrophic crashes and continues to threaten general aviation and high performance jet aircraft.

In the prior art, several de-icing and anti-icing systems have been developed to combat airbone aircraft icing. Such systems are typically classified in accordance with the means used to prevent or remove the icing--either thermal, chemical or mechanical. Furthermore, de-icing and anti-icing systems differ in their method of operation.

At present, two types of thermal anti-icing systems are currently in general use. Each type heats ice prone surfaces to a temperature sufficient to prevent the formation of ice. The two basic thermal systems are the so-called "bleed-air" and electro-thermal systems. The former uses exhaust gas from the aircraft to heat air which is then fed into cavities behind ice prone areas so as to melt the ice or prevent it from forming. Although these anti-icing systems are reliable, they consume a great deal of energy in operation.

Electro-thermal systems use resistive elements embedded within thin sheets of material placed over and/or under the ice prone surfaces. Electricity is supplied from a power source and passes through resistive elements which generates heat sufficient to melt the ice. Such electro-thermal systems likewise are power intensive. Moreover, attempts to minimize the energy consumption by melting the ice periodically, rather than continuously, often leads to "runback re-freeze" a condition whereby water from the melting ice flows to adjacent areas on the airfoil and refreezes.

Other anti-icing systems have attempted to solve the disadvantages of thermal systems. For example, so-called "weeping wing" systems have been developed. A chemical, such as an antifreeze solution, is pumped through capillaries distributed over the ice prone surfaces. The antifreeze solution prevents the ice from forming. However, in order to function properly the system must be in operation prior to the aircraft encountering icing. This limitation coupled with the environmental concerns of introducing such chemicals into the environment reduces the desirability of such systems.

De-icing systems, unlike anti-icing systems which prevent the formation of ice, remove already existing ice accretions from the surface of the aircraft. They generally exist in the form of pneumatic or electro-magnetical impulse actuators. Pneumatic systems utilize inflatable rubber bladders, such that when they expand, ice is sheared, cracked, and flaked off. The actuator part of the device is installed as a thin cap (a boot) that covers the ice prone area. Rubber pneumatic de-icer boots are widely used, but are unfortunately prone to damage from weather and foreign objects--typically, having a service life of 2-5 years. Further, they cannot effectively remove thin layers of ice less than about 0.25." One alternative pneumatic system under development utilizes a high pressure pneumatic pulse that causes the icing surface to move with a high acceleration and low displacement. Although attractive, this type of system requires a considerable amount of auxiliary equipment and, as such, is expensive and rather bulky. Further, although such systems use erosion resistant polymers, its durability falls far short of the possibility of using metals.

Electromagnetic de-icing systems have also been widely investigated. See, for example, U.S. Pat. Nos. 4,690,353 and 5,129,598 which are incorporated herein by reference. They perform in substantially the same manner as the impulse pneumatic system, but use current induced magnetic effects that results in either surface vibrations or strong pulses to effect de-icing. For example, in U.S. Pat. No. 4,690,353, Haslim et al. describe an electro-expulsive separation system in which an elastomeric covering referred to as a blanket, cuff, or boot is placed on an aircraft surface. Mutually repelling conductors are employed that distend the blanket abruptly when a current is applied, thus propelling ice from the protected surface. More specifically, current discharged into the conductors gives rise to magnetic fields of like polarity that result in an electro-mechanical excitation. However, to obtain the required expulsion force, an extremely high current flow through the conductors is required. Such a current flow may be generated by capacitively storing and then discharging a high charge through the conductors. To effect discharge, very high current rated silicon control rectifiers must be used. Such high power rectifiers are, however, bulky and costly. Moreover, to achieve such high currents for actuators of a practical size, high voltages must be used, making the system susceptible to internal shorting. At modest current levels, the expulsion force affords only a limited de-icing capability. Also, like the pneumatic boot these systems generally require an auxiliary mechanism to eliminate "auto-inflation" that occurs when polymer boots having internal hollow voids are used to make the system more effective using modest current levels.

Alternative electro-magnetic de-icing systems use induced eddy currents which act on the surface of the leading edges of metal airfoils to effect de-icing. See, for example, U.S. Pat. Nos. 4,399,967 and 5,129,598, which are incorporated herein by reference. Although electro-magnetic de-icing systems are potentially capable of removing very thin layers of ice and their performance is generally superior to pneumatic de-icers, they generally suffer from some or all of the following problems: limited distribution of the de-icing effect, low power efficiency, bulk, and limited service life.

Accordingly, there exits a need for a low cost, low weight improved de-icing system having, among other things, modest voltage and current requirements.

SUMMARY OF THE INVENTION

The present invention is a de-icing system that uses electro-magnetic actuator(s) mounted within the airfoil of an aircraft to effect de-icing. Advantageously, the actuators have low voltage and current requirements due primarily to the high number of coil windings used. Each actuator includes conductive strip(s) fabricated on a flexible dielectric sheet. The dielectric sheet is rolled into a flattened elongated tube such that the conductive strip(s) are wound into coil(s) with the axis of the winding coil(s) coincident to the longitudinal axis of the elongated tube. Importantly, the elongated shape of the tube constrains the coil windings from displacing along the longitudinal axis. Advantageously, such actuator coils, even with a high number of windings, can be actuated several tens of thousands of times without malfunctioning. The elongated tube may be selectively bonded within corrugated channels of a housing, which channels run in a spanwise and/or chordwise direction along an airfoil. Moreover, the housing may be integrated with the leading edge structure or shaped to conform to fit over the original shape of the airfoil.

Preferably, the axis of the coil windings is located substantially parallel to and beneath ice prone surfaces of an airfoil, such as the leading edges. The actuator and housing supporting it are covered with a protective semi-rigid sheet.

Preferably, the present invention includes driver electronics for each actuator or desired group of actuators. In particular, the driver electronics includes a capacitive storage bank, a silicon control rectifier switch, a charging circuit and a control module. In operation, the charging circuit charges the capacitive storage bank. When ice is detected on the airfoil, the control module triggers the SCR which in turns causes an instantaneous current to be discharged through the coil(s) of the actuator(s). The interaction between the current in the windings of the coil results in an impulsive electro-magnetic force or excitation that causes the coil to rapidly expand from its flattened shape to a more oval shape. This expansion in turn exerts an impulsive force on the surface of the semi-rigid sheet which then distends and recoils, thereby expelling any ice accretion on the surface thereof. After actuation, the coil returns to its original shape due to the elasticity of the tube, and the pressure exerted inwardly by the semi-rigid sheet.

Conventional circuit board fabrication techniques, such as masking and etching techniques, may be used to fabricate the conductive strips in a serpentine or digitated pattern. The serpentine or digitated pattern of conductive strips once rolled along a desired direction forms an elongated tube having parallel spaced apart coils. A strip of very high bond (VHB) strength transfer adhesive tape bonds the last two windings together to prevent the actuator from unwinding. With the tube flattened into an elongated shape, it is then heat treated to permanently set them so that the sides of the coils act as a spring to return it to the original shape after being actuated.

Various serpentine or digitated patterns may be used, including those having a redundancy configuration. In the event of a failure of the primary coil(s), the redundant coil(s) may be used to effect de-icing. Further, the present actuator(s) may employ and contact to a power bus comprising alternating layers of conductive and dielectric layers. Also, to enhance the electro-magnetic expulsion force of the coils, the conductive strips may have various geometrical shapes, including those varying in a taper and undulating fashion. Furthermore, low adhesive coatings may be used to enhance the efficiency of the electro-magnetic coils to expel the ice accretion.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention may be obtained by reading the following description in conjunction with the appended drawings in which like elements are labeled similarly and in which:

FIG. 1 depicts a perspective view through a cross section of a typical airfoil having an electro-magnetic expulsion de-icing system in accordance with the principles of the invention;

FIG. 2 depicts a block diagram of the driver electronics shown in FIG. 1;

FIG. 3 depicts a cross sectional view of an electro-magnetic expulsion actuator in accordance with the principles of the invention;

FIG. 4 depicts a top view of the electro-magnetic expulsion actuator of FIG. 3;

FIG. 5 depicts a cross sectional view of the electro-magnetic actuator taken along line 5--5 in FIG. 3;

FIG. 6 depicts one exemplary digitated pattern used in fabricating an electro-magnetic expulsion de-icing actuator in accordance with the principles of the invention;

FIG. 7 depicts a cross sectional view of the digitated pattern and dielectric sheet taken along line 7--7 in FIG. 6;

FIG. 8 depicts an electro-magnetic expulsion de-icing actuator formed from the serpentine or digitated pattern of FIG. 6;

FIG. 9 depicts an alternative electro-magnetic expulsion de-icing actuator formed from the serpentine or digitated pattern of FIG. 6;

FIG. 10 depicts an alternative serpentine or digitated pattern used in fabricating an electro-magnetic expulsion de-icing actuator in accordance with the principles of the invention;

FIG. 11 depicts another alternative serpentine or digitated pattern having redundant conductive strips fabricated on the opposite side of the primary conductive strips;

FIG. 12 depicts a cross sectional view of the serpentine or digitated pattern, and the dielectric sheet taken along line 12--12 in FIG. 11;

FIG. 13 depicts another alternative serpentine or digitated pattern having redundant conductive strips fabricated on the same side as the primary conductive strips;

FIG. 14 depicts a cross sectional view of the serpentine or digitated pattern, and dielectric sheet taken along line 14--14 in FIG. 13;

FIG. 15 depicts a serpentine or digitated pattern used in fabricating an electro-magnetic expulsion actuator for housing with a power bus;

FIG. 16 depicts a plan view of an electro-magnetic expulsion actuator housed within a channel and bonded to a power bus;

FIG. 17 depicts a cross sectional view of the electro-magnetic actuator bonded within a channel to a power bus and taken along line 17--17 of FIG. 16;

FIG. 18 depicts a top view of a plurality of electro-magnetic expulsion actuators bonded to a power bus;

FIG. 19 depicts a side view of a plurality of electro-magnetic expulsion actuators and power bus of FIG. 18;

FIG. 20 depicts an end view of the plurality of electro-magnetic expulsion actuators and power bus of FIG. 19;

FIG. 21 depicts an alternative geometrical shape for the conductive strips used in the coil windings of the present invention;

FIG. 22 depicts a top view of a single coil winding formed from the conductive strip of FIG. 21;

FIG. 23 depicts another alternative geometrical shape for the conductive strips used in the coil windings of the present invention;

FIG. 24 depicts a cross sectional view of a coil formed from the conductive strip of FIG. 23;

FIG. 25 depicts a perspective view through a cross section of a typical airfoil having an alternative embodiment of the electro-magnetic expulsion de-icing system in accordance with the principles of the invention;

FIG. 26 depicts an enlarged portion of the filleted region of FIG. 25; and

FIG. 27 depicts a perspective view through a cross-section of a typical airfoil of another embodiment of the present electro-magnetic de-icing system in accordance with the principles of the invention.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, the present invention is a de-icing system employing electro-magnetic actuator(s) 100 mounted within the airfoil of an aircraft. The de-icing system further includes driver electronics 110 that houses for each actuator or desired group of actuators, a capacitive storage bank 120, a silicon control rectifier (SCR) switch 130, a charging circuit 140 and a control module 145. Some or all of the components of driver electronics 110 may be mounted or integrated with the airfoil or within a channel 150. Alternatively, driver electronics 110 may be mounted within the cabin of the aircraft. Although SCRs are preferred because of their solid state nature and cost, controlled thyristors may also be used.

Each actuator 100 includes conductive strip(s) fabricated on a flexible dielectric sheet, discussed more fully below. The dielectric sheet with the conductive strip(s) are wound into coil(s) then shaped into a flattened tube. Importantly, the length and flatness of the tube is sufficient with respect to the generated electro-magnetic force to provide a lateral restraint that prevents the coil windings from telescoping. Advantageously, such actuator coils, even with a high number of windings, can be actuated several tens of thousands of times without displacing the coil windings. The axis of winding is coincident with the longitudinal axis of the elongated tube. The elongated tube is selectively bonded to corrugated channels 150 of a housing 160. Corrugated channels 150 run preferably in a spanwise direction. Housing 160 may be integrated with the leading edge structure or may be shaped to conform to fit over the shape of the airfoil. In the latter instance, housing 160 may be bonded to the airfoil surface with very high bond (VHB) adhesive tapes or by mechanical means.

In some instances, the curvature of semi-rigid 170 may be too sharp to be activated by the flat profile of actuator(s) 100. Accordingly, fracture proof plastic shims may be bonded to the actuators to transport the impulse forces to the curved areas of the airfoil, such as the nose.

Preferably, the tube-like actuator is located substantially parallel to and beneath ice prone surfaces of an airfoil, such as the leading edges. Actuator(s) 100 and housing 160 are covered with protective semi-rigid sheet 170 that is made of flexibly stiff, low weight and corrosion resistance material, such as stainless steel, aluminum, fiber composites or the like. Semi-rigid sheet 170 is preferably electrically conductive for lightening protection. Housing 160 is supported or fixed at support points 180 and is preferably fastened to housing 160 or the frame of the aircraft at points 190 that are located beyond the areas prone to icing. Likewise, this fastening may be accomplished through the use of VHB adhesive tapes or mechanical means.

It should be understood that semi-rigid sheet 170 can flex and lift off from support points 180. The actual number and location of the support points depend on the aerodynamic loading characteristics of the given airfoil. In particular, support points 180 are designed in conjunction with an appropriately stiff semi-rigid sheet 170 to counteract the aerodynamic loading. For example, the low pressure state on the upper portion of the airfoil can cause the so-called state of "auto-inflation." Also, between each support point 180 and further aft until fastening point 190, there is a void between semi-rigid sheet 170 and housing 160. Such a void allows the surface of semi-rigid sheet 170 to deflect and vibrate so as to allow de-icing along chordwise directions.

Referring to FIG. 2, in operation, charging circuit 140 charges capacitive storage bank 120. When icing is detected on the airfoil, control module 145 triggers SCR 130 which in turn causes an instantaneous current to be discharged through the coil(s) of actuator(s) 100. Various well known ice sensors may be used, such as those disclosed in U.S. Pat. No. 5,191,791, which is incorporated herein by reference. Preferably, control module 145 actuates each actuator 100 in succession. The interaction between the current in the windings of the coil results in an impulsive force on the surface of semi-rigid sheet 170 which then distends and recoils, thereby expelling any ice accretions on the surface thereof. It should be understood that portions of semi-rigid sheet 170 located in between the coils and/or support points 180 experience a force, F, acting along a direction other than the direction of the generated electro-magnetic force. This enhances the de-icing effect. Once actuated, the coil returns to its original shape due to the elasticity of the coil, particularly the sides of the windings, and the pressure exerted inwardly by semi-rigid sheet 170.

The actuators of the present invention are efficient enough to use low I² t rated SCRs. The I² t rating is a measure of the amount of energy a device is able to absorb on a low duty cycle basis. The I² t rating is given as follows: ##EQU1## where I² t is the rating of the device in amperes² /sec, t is the time, I(t) is the current as a function of time, and T is the time duration of the current.

Because the present invention uses short pulses of modest level current, several hundreds of microseconds or less in duration, little heat is dissipated. Accordingly, SCRs having a I² t rating as low as 50 may be used for many de-icing applications. Advantageously, commercially available SCRs with such low ratings, such as Model CR8AM from Powerex Co., may be mounted on printed circuits, thereby reducing the overall bulk and cost of the de-icing system.

An exemplary embodiment of an electro-magnetic de-icing actuator 300 in accordance with the principles of the invention is shown in FIGS. 3-5. De-icing actuator 300 comprises a conductive strip 310 disposed on a flexible dielectric thin sheet 320. Conductive strip 310 and dielectric thin sheet 320 are wound into a flattened elongated coil having multiple windings, thereby forming actuator 300. Dielectric thin sheet 320 prevents shorting between each adjacent winding. De-icing actuator 300 may comprise a plurality of conductive strips so as to form a plurality of parallel spaced apart coils (not shown). Shown in FIG. 4 is a top view of de-icing actuator 300.

In operation, a voltage, V, applied between the ends of the actuator coils, such as through contact tabs or terminals, causes current to flow, giving rise to current paths within the windings in mutually opposite directions as illustrated in FIG. 3 by the arrows. In particular, current in the top windings of the coil flows in one direction whereas current in the bottom windings flows in the other direction. As illustrated in FIG. 5, the resulting magnetic fields 330 and 340 within each winding bundle produces a repulsive force, F, that distends each half of the coil along a direction substantially perpendicular to the flow of current. More specifically, the actuator coil distends to the oval shape depicted by a dash line 350.

For each coil, the actuation force, F, is approximately given by; ##EQU2## where L is the length of the coil, C₁ is 4×10⁻⁷, b is the width of the conductive strip, d_(m) is the mean separation between the winding bundles, I is the current in the coil, and N is the number of turns in the coil. This approximation is only valid if the mean separation d_(m) is much greater than the thickness of the upper and lower individual winding layers.

In operation, the repeated activations tend to telescope the coil windings laterally from their functional position. To counteract this, the length of the tube, L_(tube), is judiciously chosen with respect to the length of the coil(s) so as to provide a lateral restraint that prevents the windings from displacing. This lateral restraint is also enhanced by the flattened geometrical shape of the tube. Experimentation indicates that the tube length should preferably be equal to or greater than the length, L, of the coils: L_(tube) /L≧1 provided that the tube is flattened. Those skilled in the art will readily note that in FIGS. 3 and 4, the width, b, of the coil is equal to the tube length. It is contemplated that the width of the coil, however, is most preferably a fraction of the tube length.

To better understand the performance of de-icing actuator 300, the inter-relationship among various physical dimensions of de-icing actuator 300 with respect to the generated force and the power requirement will be discussed. Those skilled in the art will readily note that the force, F, is strongly dependent on the number of turns, N. However, increasing the number of turns will also increase the mean winding bundle separation, d_(m), thereby tending to decrease the actuation force. To counteract this, it is contemplated that the conductive strips are kept substantially thin such that the term N² remains dominant.

Further, those skilled in the art will also readily note that the coil height, h, is one of the primary design considerations for a de-icing system. The coil height, h, varies with the design constraints imposed by the specific application. To maximize the force, F, the greatest number of turns, N_(max), should be used in conjunction with very thin winding layers and is given by: ##EQU3## where a_(min) is the minimum available thickness of the conductive strips, and t_(min) is the minimum available thickness of the dielectric, and h_(max) is the maximum allowable height of the coil. It should be noted that reducing the thickness of the conductive strips increases the resistance, R, of the coil because the cross-sectional area through which the current flows is reduced.

The resistance, R, has been derived to be given by: ##EQU4## where R is the resistance of the coil, R_(c) is the resistivity of the conductor, ab is the cross sectional area of the conductor, L is the inductance of the coil, μ_(o) is the permeability of free space and l is the coil length.

Further, the voltage required to generate a given current I is given by: ##EQU5## where I is the current, V is the applied voltage, Z_(o) is the characteristic impedance of the coil, C is the capacitance of the energy source and L is the coil inductance.

Accordingly, to tailor de-icing actuator 300 to the specific physical design and power constraints, the physical dimensions of the conductive strips can be judiciously fabricated in accordance with the above equations.

The fabrication of the present invention may be understood by reference to various configurations of the de-icing actuators in accordance with the principles of the invention. In these instances, the de-icing actuator consists of a plurality of coils formed from parallel spaced apart conductive strips. Referring to FIG. 6, a thin conductive film 600 is fabricated on a dielectric sheet 610. Preferably, the conductive film is made of copper due its high conductivity and low cost. Polyimide, also as Kapton, is the preferred material for dielectric sheet 610 because of its high tensile strength, dielectric constant and heat resistance. The high heat resistance advantageously allows contacts to be soldered to the conductive strips without harming the dielectric. Sheets of polyimide already deposited with copper are manufactured and sold by Sheldahl Co. under the trade name Novaclad™. Such sheets are also distributed by Rogers Co. under the trade name Flex-I-Mid™.

Conventional circuit board fabrication techniques may be used to generate a serpentine or digitated pattern of parallel spaced apart conductive strips 600. In fabricating conductive strips 600 on dielectric sheet 610, standard masking and etching techniques may be used. It should be understood, however, that other fabrication techniques that directly deposit the serpentine or digitated pattern on the dielectric may also be used and may be more cost effective.

Shown in FIG. 7 is a cross sectional view of conductive strips 600 and dielectric sheet 610. Preferably, conductive strips 600 are selectively etched such that the conductive material gradually fillets to the dielectric sheet (not shown in FIG. 7). In this manner, the stress near the edges of the conductive strips is reduced, which in turns reduces the potential for the conductive strips to crack away from the dielectric.

After the serpentine or digitated pattern is etched, the sheet consisting of conductive strips 600 and dielectric sheet 610, called the clad, is rolled up along direction 620 to form an elongated tube having parallel spaced apart coils 630 which are electrically connected in series, as shown in FIG. 8. Conductive portions 635 are used as contacts for applying a voltage to coils 630. Preferably, a dry lubricant is applied to the conductive strips to minimize friction and prevent corrosion. It should be understood that the number of coils and the spacing between them are dependent on the dimensions and spacing of the conductive pattern required for the specific application.

Alternatively, the clad may be rolled along direction 640 to form an actuator coil 650, as shown in FIG. 9. In particular, judiciously separating conductive strips 600 apart allows them to lay atop of one another when rolled, thereby forming single actuator coil 650. Of course, the separation between the conductive strips must be progressively increased along the direction of rolling to account for the fact that the perimeter of each winding coil increases with each subsequent roll. Coil 650 likewise operates in the same manner as discussed above.

Once the clad is wound, a strip of VHB strength transfer adhesive tape is used to bond the last two windings together to prevent the clad from unwinding. Next, the tube is flattened into an elongated shape so as to fit into, for example, corrugated channels 150 of housing 160, as illustrated in FIG. 1.

While held flat, the elongated tube is heat treated so that the windings on the sides of the coils and the dielectric sheet are permanently set to act as a spring to assist in returning the elongated tube to its original flattened shape after actuation. Moreover, the tendency for the coil windings to telescope out from the center is substantially eliminated by the flattening and heating. As discussed above, this tendency is substantially eliminated when L_(tube) /L is greater than or equal to one(1). Subsequently, a protective layer of Kapton adhesive tape may be applied to the outside of the elongated tube to electrically insulate and protect it from wear and tear. When actuated, the coil windings distend and slip over one another rather than stretching. Advantageously, this prevents the coil(s) from cracking due to fatigue.

Shown in FIG. 10 is an alternative serpentine or digitated pattern that may be used in the present invention. Dielectric sheet 610 and conductor strips 600 when rolled a direction 1000 form actuator coils from portions 1010, 1020 and 1030. Each actuator coil is similar to actuator coil 650 of FIG. 9, but are connected in series. The actuator coil formed from portion 1020, and 1030 differs slightly from portion 1010 in that one of coil layers 600, 1040 is itself used for the return current. Also, coil portion 1050 interconnects the actuator coil formed from portion 1020 to the actuator coil formed from portion 1030. It is contemplated that actuator coil portion 1050 is required with this serpentine or digitated patterns when three or more actuator coils are connected in series.

In an example from experimental practice, an electro-magnetic expulsion actuator consisting of four actuator coils was fabricated from copper conductive strips using the serpentine or digitated pattern of FIG. 6 and rolled along direction 620. Specifically, the conductive strips were fabricated on a 0.0005" thick dielectric sheet of polyimide with the following coil parameters: L=0.75", b=0.75", h=0.2", d_(m) =0.1", a=0.0007", and N=75. The tube length, L_(tube), is 14". As such, L_(tube) /L is ˜18.

Experimental testing indicates that a discharge supplied from a 250 micro-farad capacitor bank charged to 550 volts causes approximately 500 amperes to flow through the coils. The discharge time was approximately 100 micro-seconds. Due to electro-magnetic expulsion, a force of approximately 77 pounds of peak impulsive force is generated in each actuator coil. Installation of the experimental de-icing actuator within an airfoil, as shown in FIG. 1, verified the actuator's capability to effectively remove ice accretions at least 0.05" thick from an area of approximately 18" spanwise and 4" chordwise.

In other embodiments, the serpentine or digitated patterns may also use redundancy configurations wherein redundant conductive strips are fabricated on the same or opposite side of the dielectric sheet. Shown in FIG. 11 is an exemplary redundancy configuration depicting a serpentine or digitated pattern of primary conductor strips 1110 disposed on one side of a dielectric sheet 1120. Fabricated on the opposite side of dielectric sheet 1120 is a serpentine or digitated pattern of redundant conductive strips 1130. Preferably, redundant conductive strips 1130 are offset or staggered with respect to primary conductive strips 1110 to avoid shorting and minimize thickness. When rolled into an elongated tube, primary conductive strips 1110 and redundant conductive strips 1130 form pairs of parallel spaced apart primary and redundant coils, respectively, interlaced in an alternating fashion. FIG. 12 depicts a cross sectional view of primary and redundant conductive strips 1110, 1130 and dielectric sheet 1120. In the event of a failure of the primary coil(s), the redundant coil(s) may be used to effect de-icing.

Alternatively, primary conductive strips 1110 and redundant conductive strips 1130 may be fabricated on the same side of dielectric sheet 1120, as shown in FIG. 13 and 14. Alternative serpentine or digitated patterns may also be used to facilitate the use of a power bus integrated with, for example, a corrugated channel supporting the de-icing actuators. Referring to FIGS. 15-17, de-icing actuator coils 1525 are comprised of conductive strips 1510 disposed on a flexible dielectric thin sheet 1520. Conductive strips 1510 include terminal tabs 1530. Likewise, conductive strips 1510 and dielectric thin sheet 1520 are wound into a flattened elongated tube to form series connected actuator coils 1525 having multiple windings which form an actuator unit 1500. A power bus 1540 provides the current necessary to actuate the actuator(s) and is bonded into a corrugated channel 1505. Actuator coil unit 1500 is bonded to power bus 1540 with the exposed surface of terminal tabs 1530 electrically contacting corresponding contact tabs (not shown) of power bus 1540. Preferably, power bus 1540 and actuator unit 1500 are bonded with VHB adhesive tapes. Shown in FIGS. 16 and 17 are top and cross sectional views, respectively, of actuator unit 1500 unit and power bus 1540 bonded within corrugated channel 1505.

When redundant actuator coils are used, those coils are fabricated with terminal tabs that face outward from the contact tabs of the power bus. These latter terminal tabs are soldered to the power bus by folding them to expose their conductive surfaces to the contact portions of the power bus.

To more fully understand and appreciate the structure of power bus 1540, shown in FIGS. 18, 19 and 20 are top, side and end views, respectively of a plurality of actuator units 1500 connected to power bus 1540. Power bus 1540 comprises alternating layers of conductive and dielectric layers 1900 and 1910, respectively, such as copper and polyimide layers. Advantageously, these conductive and dielectric layers may be fabricated from the same materials used to fabricate the actuators. Conductive and dielectric layers 1900 and 1910 are bonded together with VHB adhesive tape. A common bus layer 1920 connects to the positive terminals of each actuator 1500. As such, terminal tabs of common bus layer 1920 that connect to the positive terminal tabs 1530 of actuators 1500 are insulated so as not to short to the other layers. Also, the terminal tabs are folded to expose its conductive surface upwards. Conductive layers 1900 contact corresponding negative terminal tabs 1530 of each actuator coil 1525.

Referring again to FIG. 21 is a conductive strip 2110 having undulating or repeating wide and narrow portions 2120 and 2130, respectively. When rolled, wide and narrow portions 2120, 2130 form ends and the horizontal areas of the coil windings, respectively. It should be noted that the length L₁ is just sufficient to achieve one complete coil winding and increases for each subsequent winding to account for an increase in its perimeter. With the ends of the coils widen, the above described parasitic effect is reduced because of the lower magnetic field and resistance now associated with the ends. Similarly, the narrower horizontal areas increase the electro-magnetic expulsion force because of the strengthened magnetic field. Shown in FIG. 22 is a top view of a single coil winding formed from conductive strip 2110.

Shown in FIG. 23 is a conductive strip 2310 having an alternative geometric shape used to reduce internal stress and enhance the electro-magnetic expulsion force. Conductive strip 2310 is tapered--narrowing from a width L₂ to a width ₃ L. When rolled along a direction 2320, conductive strip 2310 forms a coil 2330 having a cross-section geometry as shown in FIG. 24. Advantageously, this winding geometry permits the windings of coil 2330 to have a lower internal stress between each winding. This is so because a larger load bearing area is provided from the innermost to the outermost coil windings. The force per winding decreases from the innermost to the outermost windings due to the increasing width of the windings. This is more than compensated for by the fact that the innermost windings are more closely spaced apart. It is contemplated that a tapered geometry coil generates a greater force per current flow than a constant width coil having the same mean thickness, resistance, and number of windings.

It is also contemplated that the width of the conductive strips may be nonlinearly varied so as to yield a higher concentration of narrow innermost coil windings and wide outermost coil windings. Further, the conductive strips may use a combination of different coil geometries, including the taper and undulating geometries discussed above.

Shown in FIG. 25 is another embodiment of the present invention in which a de-icer 2570 is installed over an airfoil 2500. De-icer 2570 uses an actuator 2520 similar to that of FIGS. 8 or 9, except that the length of the coil, L, is sufficiently long so as to cover a substantial portion of airfoil 2500 along a chordwise direction. It is contemplated that actuator 2520 completely wraps around ice prone areas of airfoil 2500. The edge of de-icer 2570 has an erosion shield 2530 that covers actuator 2520. Erosion shield 2530 is fastened to airfoil 2500 at filleted region 2510 by VHB adhesive tapes or mechanical fasteners. The underside of de-icer 2570 includes an electrically insulative sheet 2540. Actuator 2520 is bonded to insulative sheet 2540 which in turn is bonded to airfoil 2500.

Preferably, actuator 2520 includes spanwise and/or chordwise conductive strips 2550 and 2560, respectively, that form corresponding coils. Spanwise conductive strips 2550 effect de-icing at or near the regions of the airfoil having sharp curvatures, such as those near the nose of the airfoil. The spanwise conductive strips minimize the hoop strain effect of the outwardly expanding winding bundles. Chordwise conductive strips located in the further aft regions are less susceptible to fatigue.

Regardless of the orientation of the conductive strips, the dielectric sheet on which the conductive strips are fabricated on expands outwardly when actuator 2520 is actuated. As such, this sheet experiences hoop strain, resulting in fatigue as well as reduced power efficiency. Shown in FIG. 26 is an enlarged portion of filleted region 2510. To counteract the hoop strain, the insulative sheet and/or the conductive strips of actuator 2520 is fabricated with corrugations 2600 at various locations formed with chordwise or spanwise conductive strips by restraining and heating treating the desired portions of actuator 2520. Importantly, when actuator 2520 is actuated, corrugations 2600 expand sufficiently to relieve the hoop strain.

It is further contemplated that the present electro-magnetic de-icing system may utilize low adhesive materials so as to optimize the mechanical de-icing effect potentially to the performance level of an anti-icer. It should be clearly understood, however, that other electro-mechanical systems, such as pneumatic systems or electro-mechanical systems, may be used instead of the present electro-magnetic actuators. As such, shown in FIG. 27 is another embodiment of the present invention similar to the above embodiments and in which a selected low adhesive coating 2750 is deposited on at least a portion of semi-rigid sheet 170. Alternatively, low adhesive coating 2750 may be deposited on the entire portion of the semi-rigid sheet depending on the desired application. The impulsive force generated by actuator(s) 100 causes semi-rigid sheet 170 and low adhesive coating 2750 to deflect and vibrate, thereby allowing de-icing to be effected along a chordwise direction. Although low adhesive coating 2750 may be applied on any region of the airfoil, low adhesive coating 2750 is preferably placed aft of the main ice pack region, as illustrated in FIG. 27. In practice, without special low adhesive coatings the main ice pack sheds cleanly. Just aft of the main ice pack, however, a grainy residual ice pack usually develops and will not shed below a given thickness, unless low adhesive coatings are used.

Low adhesive coating 2750 is preferably of the type known as synergistic coatings, such as Nedox, Tufram, Canadize, Magnadize and the like. Such coatings are generally characterized by a very hard surface layer structurally bonded to the underlying surface and formed in a manner permitting the infusion of one or more varieties of polymers or co-polymers, such as TFE's, PTE's and non-fluorinated materials.

Low adhesive coating 2750 may be applied to semi-rigid sheet 170 by electro-deposition, anodic oxidation, plasma application, or any other suitable process known in the art. The addition of polymers or co-polymers to lower adhesive properties of coating 2750 not only provides the desirable lubricity, but also provides the release properties necessary for the detachment of ice accretion. In this latter manner the ice accretion does not substantially adhere to coating 2750, thereby allowing airflow over the airfoil to readily expel the ice accretion. Although low adhesive coating 2750 alone may provide passive de-icing under certain conditions, the combination of the actuation impulse of actuator(s) 100 and the low adhesive properties of the coating improves de-icing, thereby offering a yet lower power approach that can be optimized with proper power management.

It should be understood that various other modifications will be readily apparent to those skilled in the art without departing from the scope and spirit of the present invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description set forth herein, but rather than the claims be construed as encompassing all the features of the patentable novelty that reside in the present invention, including all features that would be treated as equivalents thereto by those skilled in the art to which this invention pertains. 

We claim:
 1. A de-icing system comprising:a housing, at least a portion of which is positioned underneath an airfoil; a low adhesive coating covering at least a portion of said airfoil; and means for applying an impulsive force near or on said low adhesive coating so as to remove ice accretion thereon, said means for applying located in said housing.
 2. The de-icing system of claim 1 wherein said low adhesive coating is a synergistic coating.
 3. The de-icing system of claim 1 wherein said low adhesive coating is a coating selected from the group of Nedox, Tufram, Canadize, or Magnadize.
 4. The de-icing system of claim 1 wherein said low adhesive coating covers at least an aft portion of said airfoil.
 5. The de-icing system of claim 1 wherein said means for applying an impulsive force includes an electro-magnetic actuator.
 6. The de-icing system of claim 1 wherein said means for applying an impulsive force includes an electro-mechanical actuator.
 7. A de-icing system comprising:a housing, at least a portion of which has a channel; a semi-rigid sheet formed over said housing; a low adhesive coating covering at least a portion of said semi-rigid sheet; and means for applying an electro-impulsive force near or on said low adhesive coating so as to remove ice accretion thereon.
 8. The de-icing system of claim 7 wherein said low adhesive coating covers the aft portion of said semi-rigid sheet.
 9. The de-icing system of claim 7 wherein said mean for applying an electro-impulsive force includes an electro-magnetic actuator havinga dielectric sheet, and at least a first conductive strip disposed on a first side of said dielectric sheet, said dielectric sheet and said at least a first conductive strip being rolled into a flattened elongated tube such that at least a first conductive strip is wound into at least a first coil having an axis of winding along a first direction, said elongated tube being bonded within the channel of said housing and having a longitudinal axis coincident with said axis of winding.
 10. The de-icing system of claim 9 further comprisingmeans for applying a current through said at least a first coil, thereby causing said at least a first coil to rapidly expand so as to exert a force over a portion of said semi-rigid sheet.
 11. The de-icing system of claim 10 wherein said means for applying a current includes,means for storing a capacitive charge, means for charging said means for storing to a predetermined capacitive charge, and means connected to said electro-magnetic actuator for discharging the predetermined capacitive charge through said at least a first coil.
 12. The de-icing system of claim 11 wherein said means for discharging includes a rectifier switch.
 13. The de-icing system of claim 9 wherein said elongated tube has a length, L_(tube), along the longitudinal axis, and said at least a first coil has a length, L, along a direction perpendicular to said longitudinal axis, and wherein L_(tube) /L is sufficiently large to prevent the windings of the coil from displacing with respect to each other.
 14. The de-icing system of claim 9 wherein L_(tube) /L is greater than or equal to one. 